Conventional laser light sources include a Fabry-Perot laser light source, which uses a Fabry-Perot resonator, and a distributed feedback (DFB) laser light source, which uses a diffraction grating. These types of laser light sources produce an oscillation of laser light by amplifying light of a predetermined wavelength through resonation or diffraction.
In recent years, new types of laser light sources using a photonic crystal have been developed. A photonic crystal includes a dielectric matrix body in which an artificial periodic structure is created. Usually, the periodic structure is created by providing the matrix body with a periodic arrangement of areas whose refractive index differs from that of the matrix body (this area is called the “modified refractive index area” hereinafter). The periodic structure causes a Bragg diffraction within the crystal and creates an energy band gap with respect to the energy of light. There are two types of photonic crystal laser light sources: one of which utilizes a band-gap effect to use a point-like defect as a resonator, and the other utilizes a standing wave at a band edge where the group velocity of light becomes zero. Both of them cause an oscillation of laser light by amplifying light of a predetermined wavelength.
Patent Document 1 discloses a laser light source in which a two-dimensional photonic crystal is created in the vicinity of an active layer containing a light-emitting material. This two-dimensional photonic crystal is created from a plate-shape matrix body made of a semiconductor in which cylindrical holes are arranged periodically (in a triangular, square, or hexagonal lattice pattern or similar pattern) and the refractive index of the matrix body is periodically distributed over a two-dimensional area. This period is adjusted so that it equals the wavelength within the medium of light generated in the active layer by injecting carriers from the electrode. Therefore, two-dimensional standing waves are created within the two-dimensional photonic crystal, whereby the light is intensified to realize a laser oscillation.
FIG. 1 schematically shows standing waves created within the two-dimensional photonic crystal disclosed in Patent Document 1. FIG. 1 shows only a one-dimensional aspect of the standing waves in a specific direction (called the “x-direction” hereinafter) within the crystal surface. If, for example, holes are arranged in a square lattice pattern, another standing wave occurs in the direction perpendicular to the x-direction. For the electric field, the standing wave has two modes: the first mode has a node at the holes 12 of the two-dimensional photonic crystal 11, whereas the second mode has an antinode in the same location. For a given hole 12, when an axis (“z-axis) passing through its center is defined as the axis of symmetry, the first mode is anti-symmetric with respect to the z-axis, whereas the second mode is symmetric. From the viewpoint of the coupling to the external plane waves, the distribution function of the plane waves propagating in the z-direction takes a uniform value with respect to the x-direction. In contrast, with respect to the axis of symmetry, the distribution function is an odd function for the anti-symmetric mode or an even function for the symmetric mode. Suppose that the two-dimensional photonic crystal has an infinite size. Then, if the mode is symmetric, the first-order diffracted light is emitted in the direction perpendicular to the surface because the overlap integral between the symmetric waves and the external plane waves is not zero. In the anti-symmetric mode, since the overlap integral between the generated waves and the external plane waves is zero, the emission of the first-order diffracted light in the direction perpendicular to the surface does not occur due to the interference. Thus, the anti-symmetric mode of light cannot be extracted in the direction perpendicular to the surface.
In practice, two-dimensional photonic crystals have a finite size. Therefore, even the anti-symmetric mode of light has its symmetry broken, so that the light can be extracted in the direction perpendicular to the surface. However, even in this case, the intensity of light to be extracted in the direction perpendicular to the surface is weakened by receiving the effects of interference.
In order to suppress such interference effects and improve the efficiency of extracting light in the direction perpendicular to the surface, there have been studies made for breaking the symmetry of a refractive-index distribution within a plane of a two-dimensional photonic crystal. Patent Document 2 discloses a surface-emitting laser light source having a two-dimensional photonic crystal in which the lattice structure has translational symmetry but does not have rotational symmetry so that the symmetry within a plane parallel to the matrix body is broken. For example, this type of symmetric structure can be obtained by arranging holes, which are refractive-index areas, in a square lattice pattern and creating a plane shape (i.e. shape of a cross section disposed in parallel to the two-dimensional photonic crystal) of each hole into an equilateral triangle. In this case, the lattice has four-fold rotational symmetry and the holes have three-fold rotational symmetry, where rotational symmetry is not consistent between them and this suggests that rotational symmetry is not present in the crystal as a whole. There is another method, in which two holes with a plane shape perfectly round and different diameters are adjacently arranged in each lattice point of a square lattice. This method does not provide rotational symmetry in the lattice point, which means the crystal as a whole does not have rotational symmetry. Since the lattice structure of a two-dimensional photonic crystal in these laser light sources has a lower degree of symmetry than the lattice structure of FIG. 1, it is possible to suppress the interference effects of the anti-symmetric mode of light and increase the intensity of light to be extracted in the direction perpendicular to the surface more than conventional intensities of light.    [Patent Document 1] Unexamined Japanese Patent Application Publication No. 2000-332351 (Paragraphs [0037] to [0056], FIG. 1)    [Patent Document 2] Unexamined Japanese Patent Application Publication No. 2004-296538 (Paragraphs [0026] to [0037], FIGS. 1 to 5)